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Technischer Bericht NTB 85-11

Sondierbohrungen Böttstein, Schafisheim, Weiach, Kaisten, Riniken Langzeit-Beobachtung der Tiefengrundwässer

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Between 01.01.19841st January 1984 and 30.07.1985, long-term observations of the deep groundwater in crystalline rock and in selected aquifers in the sedimentary cover began in five of the completed exploratory boreholes of the Nagra deep drilling programme. This report includes the records up to the 30.09.1985.

The support for the whole multi-packer system is a 2 7/8" or 2 3/8" tubing string into which the individual packers are integrated. The latter are inflated with water from the surface through one or more inflation lines and maintained under pressure.

Observation pipes lead from the hydraulically isolated zones upwards and, in the case of artesian conditions, are linked to manometers and valves. If the conditions are non-artesian, measurement of the free water-level is done using electric contact water-level tape.

All test results available to date were taken into consideration in dividing the boreholes into hydraulically isolated zones. In addition to the results of packer tests, the field results of the fluid logging and hydrochemical data were available.

All multi-packer systems provided important results on the pressure conditions of groundwater in vertical profiles. It should be noted that these long-term data are no longer subject to the disturbances. associated with drilling operations.

As some intervals in the open borehole reach lengths of up to several hundred meters, the piezometric water levels registered represent mixed values which occur over the whole length of the zone.

After the individual zones are sealed, the hydrographs of the water levels show very different behaviour depending on borehole and aquifer type. The decisive factors are, on the one band, the previous hydraulic history of the aquifer and, on the other band, its hydraulic conductivity, storativity, the prevailing water pressure and the extent of the sealed interval volume.

In the Böttstein borehole, all zones in the multi-packer contain artesian groundwater. The values measured allow the crystalline section (including Buntsandstein) to be divided into two from a hydrogeological viewpoint: down to a depth of approx. 1'000 m, the data indicate the presence of permeable zones in which pressures of 1.5 – 2.0 bars (relative to ground surface) increase only insignificantly with depth.

Below this depth, on the other hand, the pressures registered at ground surface increase with increasing depth up to 7 bars with generally low conductivity of the formation, which corresponds to a water-level of approx. 70 m above ground.

In the Schafisheim borehole, the water is artesian in the Muschelkalk aquifer, with a level of approx. 7 – 8 m above ground surface. In the upper part of the crystalline basement, a permeable zone determines in- and outflow and therefore the piezometric levels. Given its strongly reduced permeability, the Buntsandstein here cannot play the role of an independent aquifer. In the crystalline rock with its fissured sections, water-levels remain approx. 65 m below ground without notable exception. Only below 1'910 m does the hydraulic head apparently increase by a few meters.

In the Weiach borehole, the isolated intervals show an interesting pressure distribution. The water-level in the Muschelkalk is approx. 13 m below ground surface and the other groundwaters are artesian. Practically the same pressure of 4.5 bars is measured for the waters of the Buntsandstein and the crystalline rock below 2'210 m, corresponding to a hydraulic head of approx. 45 m above ground surface. However, the intervening Permocarboniferous and the upper section of the crystalline rock from 2'065 to 2'210 m is characterised by a wide range of pressure maxima from 6 to over 7 bars. A reasonable working hypothesis is that gas production in the Permocarboniferous trough causes the high pressure in the sediment.

In the Kaisten borehole, the groundwater levels increase almost uniformly from 25 m below ground surface in the Buntsandstein down to a depth of 700 m: here, a value of 30 m above ground surface is reached. In the crystalline section below, the pressures hardly increase at all. This results in a vertical pressure distribution which is in accordance with expectations as the borehole is situated in the discharge area of the Rhine valley.

The water-levels in the observation system of the Riniken borehole were still variable at the completion of this report on 30.09.1985. As the only aquifer, the Schilfsandstein (Keuper) has artesian water at a pressure of somewhat more than 7 bars. On the other hand, the hydraulic heads of the remaining stratigraphically deeper aquifers do not reach ground surface. Subject to the future development of the hydrographs, it is generally true that the piezometric level decreases from upper to lower levels.

While the Böttstein system has been functioning without any problems for more than 20 months, there were significant failures in Schafisheim and Weiach after only a few weeks.

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